The present invention relates to electric motors and generators and in particular to adjusting the orientation of fixed magnets in a rotor to obtain efficient operation at various RPM.
Brushless DC motors are often required to operate at various RPM but can only achieve efficient over a limited RPM range.
Further, generators and alternators are often required to operate over a broad RPM range. For example, automotive alternators operate at an RPM proportional to engine RPM and windmill alternators operate at an RPM proportional to wind speed. Unfortunately, known alternators generate electricity at a voltage proportion to RPM. Because RPM cannot be controlled, other elements are required to adjust the output voltage, adding inefficiency, complexity, and cost to the alternator systems.
Some designs have attempted to broaden RPM range using “field weakening” to allow the motor base speed (Kt or torque sensitivity) to be wound to be efficient at very low RPM, which is proportional to torque (lower RPM higher torque and vice versa), and to obtain efficient high RPM operation. Such field weakening can be in an Interior Permanent Magnet Synchronous Motor (IPMSM) or AC synchronous induction motors three to four times base speed with reasonable efficiency at high RPM but a motor with a ten times base speed RPM would have two and one-half to three and one-half times the starting torque of an AC motor. Unfortunately, field weakening with conventional methods can sacrifice efficiency and increase the complexity of controller algorithms and software.
In a generator/alternator application, the output voltage is proportional to magnetic flux strength requiring an inverter or separate electromagnetic exciter coil in automotive alternators that are only 60-70% efficient because of the very wide RPM range the alternators must operate over. Similar issues are present in wind power generation where variations in wind speed encountered resulting in operating inefficiencies.